1. Field of the Invention
The present invention relates to a wafer airtight keeping unit and a facility thereof for keeping temporarily a wafer cassette in a clean room for handling a semiconductor wafer.
2. Prior Art
A conventional article keeping facility in a clean room is disclosed in Japanese Publication Patent No. 1-41561. The article keeping facility comprises article keeping shelves having a plurality of partitioned accommodating spaces arranged vertically and laterally and a loading apparatus provided with an article taking-in-and-out mechanism which is movable vertically and laterally along a given route which is laid out on a central passage arranged between the article keeping shelves.
The article keeping facility of this type is provided with an air supply device, which is disposed under the article keeping shelves, and the air supplied from the air supply device is cleaned through a filter and blown from the rear side of the article keeping shelves toward the loading apparatus so that dust is prevented from floating or staying at the side of the article keeping shelves.
Since the loading apparatus mounts a lifting guide device and a lifting driving device for guiding the article taking-in-and-out mechanism on its self-propelled carriage, dust is generated due to abrasion and spreads toward the article keeping shelves and attached to the semiconductor wafers etc. without drawing the dust under the floor side and removed therefrom. Accordingly, it is necessary to entirely cover both the lifting guide device and the lifting driving device while an air suction device is mounted at the lower side of the self-propelled carriage, whereby the dust generated within the cover is drawn downward and removed by the air suction device through the floor of the carriage.
As mentioned above, the floating dust is prevented from attaching to the articles (semiconductor wafers accommodated in wafer cassettes) on the article keeping shelves by forming a clean air current from the rear portion of the article keeping shelves toward the central passage.
As is evident from the conventional article keeping facility, a measure to prevent the dust is taken since the semiconductor wafer is apt to be easily damaged by the dust. However, it is also necessary to take into consideration formation of an oxide film caused by a spontaneous oxidation of a semiconductor IC involved in high integration of the semiconductor IC.
FIGS. 14 and 15 are graphs disclosed in the "Super LSI ultra clean technology symposium" (Nov. 19, 1990) which FIG. 14 shows a relation between an oxide thickness and time involved in forming of the oxide film on the surface of a silicon semiconductor wafer caused by spontaneous oxidation and FIG. 15 shows a relation between resistivity and time involved in forming oxide film. FIG. 14 shows that growth rate of the oxide film is increased after the lapse of 100 to 200 minutes while FIG. 15 shows that resistivity is abruptly increased in the situation of non-cleaning of the IC after the lapse of about 50 minutes.
There is such a problem that high integrated IC is difficult to be manufactured because the oxide film is generated thereon by the spontaneous oxidation since the semiconductor wafer is normally kept for several hours to several days.
As the semiconductor integrated circuit is further advanced to high integration, it is necessary to convey and move the wafer in the atmosphere of inert gas such as N.sub.2 gas for restricting the oxide film caused by the spontaneous oxidation from growing. At present, the N.sub.2 gas atmosphere is required to have a concentration of at least 10 ppm of O.sub.2 and 100 ppm of H.sub.2 O.
To meet the demand of this atmosphere, the process of manufacturing a semiconductor device, which is conventionally carried out in a specific design clean room, has come to employ a mechanical interface device which is disclosed in Japanese Laid-Open Publication Patent No. 60-143623 in which the semiconductor device is manufactured under the N.sub.2 gas atmosphere.
Accordingly, it is necessary to keep temporarily therein the keeping unit (stocker), which keeps the semiconductor wafer always under the N.sub.2 gas atmosphere during the process of manufacturing the semiconductor device.
However, the conventional stocker provided with a door is not airtight enough against a surrounding atmosphere so that an entire stocker per se communicates with the surrounding atmosphere every time the door is open, therefore, a large amount of N.sub.2 gas had to be introduced into the stocker every time the stocker communicates with the surrounding atmosphere. Accordingly, there was a problem that a turbulence is generated in the stocker every time the large amount of N.sub.2 gas is introduced into the stocker, which permits the dust in the stocker to float in the air, permits a concentration of the N.sub.2 gas in each cassette housing room nonuniform and make the arrangement of the stocker complex and large sized.
It is therefore a first object of the present invention to provide a wafer keeping facility which is capable of restraining the growth of the oxide film caused by the spontaneous oxidation during the keeping of the ICs.
It is a second of the present invention to provide a wafer airtight keeping unit which is capable of airtightly sealing the unit except a cassette housing room against an external atmosphere when a wafer cassette is taken in and out from the unit, and is capable of restricting the N.sub.2 gas concentration from varying and the dust from floating in the other portions of wafer airtight keeping unit when the cassette is taken in and out from the unit, for thereby increasing the manufacturing yield of the semiconductor device compared with the conventional unit.